Users of the Ordo will see that today the prayer Fidelium is added in penultimate place (as are the corresponding prayers to the secrets and post-communions) in the appointed collects of the day.
The prayer Fidelium is taken from the 'Daily' Mass of the Dead and when it is prescribed for the first 'free' day of each month, with the exception of the month of November, Advent, Lent and Paschaltide. In Cathedral and Collegiate churches the Mass in choir is a Requiem and not that which goes with the Office of the Day. In other Masses that are not a Requiem, and in all other places, the prayer Fidelium etc is added to the appointed collects as described above.
Prior to the changes in the rubrics following the reform of 1911-13 on the first day of each month outside of Advent, Lent and Paschaltide free of a feast of nine lessons (so on simple feasts etc) in Cathedral and Collegiate churches two Masses were sung when a simple feast, ferial day with a proper Mass, resumed Sunday occured, one of the Office of the day and a Requiem. In other churches the collect Fidelium was added after the collect of the day. This venerable praxis of remembering the Dead was abolished in the 1955 changes.